1. Technical Field
This invention relates to a driving unit that drives a lens frame holding a lens by a screw engagement structure formed by a lead screw and a nut, in the direction of an optical axis.
2. Background Art
A well-known, conventional lens-driving unit includes a base (or a case), a lens frame holding a lens, a guide shaft fixed to the base so as to guide the lens frame in a direction of an optical axis, a motor fixed to the base, a lead screw connected directly to the motor so as to be rotated and driven, a nut that is held unrotatably with respect to the lens frame and detachably in the direction of the optical axis and that is screwed onto the lead screw, a guide groove (rotation stopping groove) formed integrally with the base so as to slidably guide projections of the nut in the direction of the optical axis, and a coil spring that urges the lens frame toward the nut. One of the projections of the nut is unrotatably held by being engaged with the rotation stopping groove of the lens frame, whereas the other projection of the nut is engaged with the guide groove of the base with slight looseness so as to be guided in the direction of the optical axis. The lead screw is rotated by the motor, and the nut is advanced in a screwed manner, and, as a result, the lens frame is moved in the direction of the optical axis (see Unexamined Japanese Patent Publication No. 2005-24968, for example).
However, in the conventional unit mentioned above, the projection of the nut is engaged directly with (or is brought into direct contact with) the rotation-stopping groove of the lens frame. Therefore, there is a fear that the projection of the nut will repeatedly strike against the rotation stopping groove while making a bumpy movement, and, as a result, the lens frame will be vibrated in a direction perpendicular to the optical axis, because of a load fluctuation caused by an assembly tolerance between the lead screw and the nut or caused by a machining tolerance in a screw part formed on them, or because of the rotational driving force of the motor and a reaction thereagainst.
Therefore, in a CD or DVD optical pick-up unit provided with this unit, there is a fear that, when the lens frame is moved in the direction of the optical axis during reading or writing of data recorded on a disk, the optical axis of the lens will be irregularly varied, thus making it difficult to keep a focus located on the disk constant.